This invention relates to an electrodeionization process wherein liquid to be purified is passed through an ion depletion compartment containing substantially uniform sized anion resin beads and uniform sized cation resin beads under the influence of a polar field in order to effect ion transfer from the liquid in the ion depletion compartment to liquid in an ion concentration compartment.
The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture.
The most well known processes include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration and ion exchange. A lesser known method is electrodeionization, occasionally mistermed filled cell electrodialysis.
The first apparatus and method for treating liquids by electrodeionization was described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. The first of these patents describes an apparatus and process for the removal of ions within a liquid mixture in a depletion chamber through a series of anionic and cationic diaphragms into a second volume of liquid in a concentration chamber under the influence of an electrical potential which causes the preselected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transferred ions and carries them in concentrated form. The second of these patents describes the use of macroporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic diaphragms. This ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membranes for the movement of ions.
The term "electrodeionization" refers to the process wherein an ion exchange material is positioned between anionic and cationic diaphragms. The term "electrodialysis" refers to such a process which does not utilize ion exchange resins between the anionic and cationic diaphragms. Illustrative of other prior art attempts to use a combination of electrodialysis and ion exchange materials or resins to purify saline from brackish are described in U.S. Pat. Nos. 2,794,770; 2,796,395; 2,947,688; 3,384,568; 2,923,674; 3,014,855 and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517 and 4,284,492.
A commercially successful electrodeionization apparatus and process is described in U.S. Pat. No. 4,632,745. The apparatus utilizes ion depletion compartments containing an ion exchange solid composition and a concentration compartment which is free of ion exchange solid material. The electrodeionization apparatus includes two terminal electrode chambers containing an anode and a cathode respectively which are utilized to pass direct current transversely through the body Of the apparatus containing a plurality of ion depletion compartments and ion concentration compartments. In operation, the dissolved ion salts of the liquid are transferred through the appropriate membranes from the ion depletion compartments to the ion concentration compartments. The ions collected in the ion concentration compartments are removed through discharge outlets and are directed to waste.
At the present time, anion exchange beads and cation exchange beads having a substantially uniform bead size are available for use in ion exchange processes that is, in processes wherein an ion dissolved in a liquid can be exchanged with an ion ionically bonded to the beads. Prior to their use, the regenerated beads must be washed with water in order to remove leachable components in the beads such as, residual regenerant, total organic carbon components and polymeric leachables such as sulfonated polystyrene. These leachable components must be removed prior to use in order to prevent contamination of the aqueous medium being treated or contaminating downstream apparatus or processes. It has been found that when utilizing the substantially uniform size ion exchange beads, the rinse or flush out time required can be as low as about 1/3 the flush out time required to clean commercially available beads having a non-uniform bead size. By the phrase, "flush out time" as used herein means the time required to obtain a quality improvement of effluent water contacting the beads before the water quality reaches a substantially constant level. Prior to the present invention, a substantially uniform sized ion exchange beads have not been utilized in an electrodeionization process.